Production of electrical oscillations by means of thermionic valves



April 21, 1925. V N. LEA

, PRODUCTION OF ELECTRICAL OSCILLATIONS BY MEANS OF THERMIONIC VALVES Filed May 10, 1921 3 Sheets-Sheet 1 FIG.

pril 21, 1925.

N. LEA

PRODUCTION OF ELECTRICAL OSCILLATIONS BY MEANS OF THERMIONIC VALVES Filed May 10, 1921 3 Sheets-Sheet 2 iii 67775? JVWW/(ll/ Zea,

April 21, 1925. 1,534,970

I N. LEA PRODUCTION OF ELECTRICAL OSCILLATIONS BY MEANS OF THERMIONIC VALVEs Filed May 1 1921 3 Sheets-Sheet :5

Patented Apr. 21, 1925 NORMAN LEA, OF LONDON, ENGLAND.

PRODUCTION OF ELECTRICAL OSCILLA'IIONS BY IvIEANS 0F THERMIOIFIG VALVES.

Application filed May 10,

To all 'whom it may concern: Be it known that I, NORMAN LEA, a subjoct of the King of Great Britain, residing in London, England, have invented certain new and useful Improvements in and Relat ing to the Production of Electrical Oscillations by Means of Thermionic Valves, of which the following is a specification.

It is well known that if the elements of a. thermionic valve having an incandesced filament (or other source of free electrons), a grid and an anode be connected in circuits having inductance and capacity, electrical oscillations can be produced if there is mutual inductance or capacity between the grid-filament and the anode-filament circuits, and it has been proposed to control the amplitude of such oscillations by diverting energy from the grid circuit by means of a second three-electrode electron discharge device connected with the grid circuit of the first device, the amount of energy so diverted and the resulting amplitude of the oscillations being varied by varying the potential of the grid of the second valve or electron discharge device.

The electrical oscillations in a simple selfoscillating valve system will in general quickly build up to their maximum or steady amplitude which is determined by an 1n herent limitation imposed on the power input to the oscillating circuits, as for example, by the form of the anode current characteristic: of the valve.

The object of the present invention is to provide means for varying at will the point at which the input power reaches its steady value without changing the valve character'- istic, and in this way to control indirectly the amplitude of the oscillations produced.

It is clear that the oscillations would be prevented building up beyond a given point, if above this point no suitable increase of oscillating potential is applied to the grid, and according to the present invention the oscillating potential applied to the grid is prevented rising above any desired limit by makinguse of the rectifier or other two-electrode equivalent device having a sharp bend in its-voltage-current curve, the current through the rectifier being used to produce in the grid circuit counter E. M. F.s of the oscillating frequency. In the arrangement illustrated in the accompanying drawings the device employed is a two-electrode ther- .mionic valve connected in series with a 1921. Serial No. 468,460.

source of E. M. F. across the coupling coil or other device supplying the oscillating voltage to the grid of the oscillating valve,- but other devices such as un'directional conducting devices, may be substituted for the two-electrode valve so long as a sharp bend in their voltage-current curve is involved.

In the drawings illustrating the invention Fig. 1 is a diagram illustrating the building up of oscillations in a simple oscillating.

of the characteristic curve, being limited,

however, by the flattening of the characteristic at A and B. Fig. 2 shows an arrangement involving a two electrode valve for controlling the amplitude of these oscillations, in which an output coil 4 is coupled to a grid circuit coil L which is connected in series with a resistance R and battery E between the grid and filament of a threeelectrode thermionic valve or equivalent electron discharge device. In a derived circuit between grid and filament there is connected a two-electrode valve 2 in series with a variable source of potential e. A source of direct current 3 shunted by a condenser 6 supp-lies the anode circuit of the valve.

The operation of this arrangement is illustrated diagrammatically in Fig. 3- which shows the normal characteristic of the valve with a grid potential of E, the normal oscillation of grld potential A, and the reduced oscillation B consequent upon the introduction of the control device, the amplitude of this reduced oscillation being determined by the value of the variable E. M. F. e in Fig. 2. The voltage of anode a of valve 2 is normally negative and the oscillations in coil L build upto acertain value approximately equal to the negative potential on a-. The tendency for the oscillating potentials across L to increase further is counteracted by the action of valve v2 which begins to conduct.

The high frequency potentials of anode a:

cause high frequency currents to flow in resistance R, producing back E. M. F.s opposing those due to coil L.

By adjusting the negative potential on anode a the amplitude to which the oscillating potentials in coil L are allowed to build up may be varied, and consequently by vary ing the value of e the high frequency output of the valve 1 may be varied.

By connecting the secondary 7 (Fig. 2 of a microphone transformer 8, preferably shunted by a condenser 9, in series with the battery 6 which need not be variable, the control of the amplitude of the oscillations may be effected Inicrophonically and the arrangement used for Wireless telephony.

Having thus described the nature of the said invention and the best means I know of carrying the same into practical effect, I claim 2- Wireless modulation system comprising a self-oscillating three-electrode valve and a two-electrode valve connected across the grid and. filament thereof, means for applying a fixed negative potential to the anode of the said two-electrode valve, means for applying modulating potentials to the said anode, a, resistance connected in series with the two-electrode valve and means for producing high frequency potentials acrom said resistance opposing those normally applied to the grid of the three-electrode Valve to produce self-oscillation.

In testimony whereof I have signed my name to this specification.

Y NORMAN LEA. 

